The present invention relates generally to a photovoltaic energy system and more particularly to systems and methods for controlling a ramp rate in a photovoltaic energy system.
Photovoltaic energy systems are used to convert solar energy into electricity using solar panels or other materials that exhibit the photovoltaic effect. Large scale photovoltaic energy systems include a collection of solar panels that form a photovoltaic field. The power output of a photovoltaic energy system is largely dependent upon weather conditions and other environmental factors that affect solar intensity. Changes in solar intensity can occur suddenly, for example, if a cloud formation casts a shadow upon the photovoltaic field.
Unpredictable and large changes in power production from grid scale photovoltaic fields can be problematic for utilities since they must maintain a precise match between electrical energy generation and customer demand. Many utilities use spinning reserves or other traditional power-generation systems to compensate for this volatility. However, these systems can be expensive to operate and maintain and often cannot respond quickly to sudden changes in photovoltaic energy production. As a result, some utilities and government entities mandate that any photovoltaic energy system supplying power to the energy grid must comply with a ramp rate. The ramp rate defines a maximum rate of change in power output provided to the energy grid by the photovoltaic energy system.
In order to comply with the ramp rate, a photovoltaic energy system must not increase or decrease its photovoltaic power output at a rate that exceeds the ramp rate (e.g., 10% of rated power capacity/min). If this requirement is not satisfied, the photovoltaic energy system may be deemed non-compliant and its capacity may be de-rated. This directly impacts the revenue generation potential of the photovoltaic energy system. Additionally, complying with ramp rate requirements may be critical to maintain proper operation of the power grid for locations with large renewable portfolios, such as island nations.
Some photovoltaic energy systems use stored electrical energy to comply with ramp rate requirements. Energy from the photovoltaic field can be stored in a battery and discharged from the battery to smooth sudden drops in photovoltaic power output. However, the battery capacity and performance characteristics needed to satisfy ramp rate requirements can be substantial. For example, the required battery capacity may be approximately 30% of the capacity of the photovoltaic energy system and the battery must be capable of discharging rapidly. High-performance batteries and associated components can be very expensive for some photovoltaic energy systems. It would be desirable to provide a photovoltaic energy system that can comply with ramp rate requirements without requiring expensive and high-performance electrical power storage and discharge components.